The use of various types of label or reporter molecule for monitoring ligand binding reactions is well-established. Some of the earliest examples of these approaches involved the use of radioactive isotopes (radioimmunoassay, RIA) and enzymes (enzyme linked immunosorbent assay, ELISA). More recently, luminescent end-points such as those involving the use of fluorescent molecules and chemiluminescent molecules have been described. All these systems rely on the ability to incorporate the label into a ligand and to detect the binding of the labelled ligand to its binding partner. Generally, these procedures require the physical separation of bound and unbound labelled ligand in order to establish the presence or absence of the binding partner. However, many examples exist in which the binding of the labelled ligand to its binding partner results in a change in the chemical or physical properties of the label itself such that no prior separation of the binding complex is required. Examples of this include inhibition of enzyme activity upon binding of an enzyme-labelled ligand (KE Rubenstein et al (1972) Biochem Biophys Res Comm, 47, 846), change of fluorescence polarisation characteristics upon binding of a fluorescent labelled ligand (W B Dandliker et al (1973) Immunochemistry 10, 219) and, more recently, change in chemical reactivity of chemiluminescent labels upon nucleic acid duplex formation (U.S. Pat. No. 5,283,174).
These methods have formed the basis of numerous classes of binding assay such as immunoassay, nucleic acid hybridisation assay and receptor binding assay. The subsequent techniques have been applied to a wide range of applications including for example the detection of viruses in blood samples, the measurement of hormone levels, the detection of genetic mutations and the detection of contaminants in the environment.